COURSE of the MONTH

Validation of viewstate MAC failed

I have hosted my application on shared server. And its working fine. But when i leave the page idle for about 20 minutes or so and then click any button then i got this error.
"Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. "

I didnt used any viewstate in the page. And i also had only one FORM tag in the page.
I have tried to put the "EVENTVALIDATION" to FALSE but it didnt work either.